This invention relates generally to a process for producing coke, and particularly to a process for producing premium-grade needle coke.
Needle coke such as that described in U.S. Pat. No. 2,775,549 is in high demand, principally as a raw material for graphite electrodes used in the steel industry. Premium grade needle coke, which is differentiated over common grade needle coke by a higher bulk density and a lower coefficient of thermal expansion (CTE) of its graphitized product, is in especially high demand. High bulk density and low graphitized product CTE are necessary characteristics of needle cokes used in the manufacture of heavy duty graphite electrodes capable of conducting large electrical currents at high temperatures.
Needle coke is traditionally manufactured in two steps. First, green (uncalcined) needle coke is prepared from petroleum residuum by a specialize delayed coking process such as that disclosed in U.S. Pat. No. 4,075,084. The green needle coke is then calcined at temperatures between about 2,000.degree. F. and 3,000.degree. F. to yield the final needle coke product.
A persistent problem with traditional needle coke manufacturing methods is their tendency to produce a large percentage of coke fines (i.e. coke particles which are sufficiently small to pass through a screen of about a No. 6 mesh). A needle coke with a preponderance of fines is unsuitable for electrode manufacture and is, therefore, much less valuable than a needle coke with a preponderance of larger particles. Thus, to the needle coke manufacturer, a large fines production means a substantial loss in revenue.
Needle coke fines can be produced in the manufacture of needle coke by several mechanisms. For many manufacturers, the predominant mechanism is the degradation of green needle coke particles during calcination. Green needle coke is considerably more friable than calcined needle coke. During the early stages of calcination, the mechanical agitation of the calcining apparatus (usually a rotary kiln) crumbles much of the green coke into tiny fragments. For those manufacturing processes which produce a highly friable green needle coke, fines production during calcination is often very large.
A need exists, therefore, for a needle coke manufacturing method which produces needle coke without producing an inordinate quantity of fines.
Consequently, an object of the invention is to provide a superior method for producing needle coke while producing fewer fines.
A further object of the invention is to provide a superior method for producing premium-grade needle coke from a highly friable green needle coke.
A still further object of the invention is to provide a superior method for reducing the friability of green needle coke.
A still further object of the invention is to provide a superior method for treating green needle coke so as to produce calcined needle coke having a bulk density which is greater than that of calcined needle coke produced by conventional treating methods.
A still further object of the invention is to provide a superior method for treating green needle coke so as to produce calcined needle coke having, when graphitized, a coefficient of thermal expansion which is less than that of calcined and graphitized needle coke produced by conventional treating methods.
These and other objects and advantages of the invention will become apparent to those skilled in the relevant art in view of the following description of the invention.